Polarization insensitive split square ring resonator based epsilon-negative and near zero refractive index metamaterial for S, C, and X frequency bands satellite and radar communications

This study has investigated the impact of inverse G-like shape resonators that exhibited epsilon negative (ENG) and near-zero refractive index (NZI) properties for multi-band wireless communications applications. The electrical measurement of structure is 0.118λ × 0.118λ × 0.021 λ, which is calculated at 3.94 GHz. FR-4 is used as a substrate layer, and the resonator is designed on it. This structure is manifested in the ENG and NZI characteristics within the frequency range of 3.8–4.17, 7.68–8.54, 10.67–11.36 GHz, and 4.07–4.15 and 8.29–8.37 GHz, respectively. This study also manifests the polarization insensitivity nature of 0°–90°, and the incident angle is investigated up to 60° for both TE and TM modes. The proposed structure achieves triple resonance at 3.94 GHz, 8.08 GHz, and 11.17 GHz, respectively, included in the S, C, and X frequency bands. The CST Microwave Studio 2019 software is conducted to design, develop, perform analysis, investigate electromagnetic properties, and extract effective medium parameters. The Advanced Design Software (ADS) is used to model the equivalent circuit of the unit cell. The simulated, measured, and ADS results verified the scattering parameter performance. The EMR value of the structure is 8.47, indicating the structure's compactness. The compact design with simplicity, ENG, and NZI properties make the proposed structure significant for microwave application, mainly to enhance the antenna bandwidth and gain filter design. ENG and NZI properties the operation frequency stability and efficiency for low earth orbit nanosatellite communications.


Design procedure and simulation setup of the structure
The anticipated structure is designed with a compact dimension of 0.118λ × 0.118λ × 0.021 λ; λ is calculated at a 3.94 GHz lower resonance frequency. The anticipated structure is designed on a 1.60 mm FR-substrate; the loss tangent and dielectric constant values of FR-4 are 0.02 and 4.4. The perfect electric conductor (PEC) is used to design the resonator of the structure. The anisotropic structure is presented in Fig. 1a, and the layout of the designed resonator is depicted in Fig. 1). The structure contains an I-like shape at the center of the resonator, two G-like shapes inversely placed at the sides of the I-like shape, and SSRR, all of the resonating elements adjacent to the metal line. The dimension of the I-like shape is significant in shifting the structure's resonance frequency.The trial and error method is used to choose different split gaps of the resonator so that the unit cell can provide maximum resonance. Table 1 is presented all of the necessary parameters with their values. The CST studio suite 2019 is numerically performed over 3-12 GHz. The transmission and reflection coefficient (S 11 , S 21 ) is observed to ensure effective responses included in the S, C, and X frequency bands. Figure 2 is depicted the structure simulation setup . The X-axis and Y-axis are instigated as PEC and PMC, where all the planes are perpendicular to each other. The electromagnetic field is applied through Z-axis. www.nature.com/scientificreports/ Development of the anisotropic structure . Several types of design are investigated in this study to get the electromagnetic properties of anticipated structure the. Figure 3 is demonstrated the different configurations of the structure, where Fig. 4a and b are shown the response of S 11 and S 21 , respectively. In step1, a square ring resonator (SRR) is designed, for instance, to provide a single resonance of S 21 at 6.66 GHz, as shown in Fig. 4b. In step 2, an inverse G-like shape is loaded at the left position of the SRR; this configuration subsidizes two resonances of S 11 at 6.30 GHz and 9.10 GHz, as shown in Fig. 4a, where the resonance of S 21 is located at the frequency of 6.36 GHz and 9.05 GHz, respectively is shown in Fig. 4b. Then metallic strip connects the inverse G-like shape and SRR. In step 3, another G-like shape is loaded at the right portion of the SRR, which causes shift resonances of S 11 and S 21 toward the lower frequency at 5.40 GHz and 5.47 GHz, presented in Fig. 4a and b. After that, in step 4, two split gaps are introduced at the top and bottom of the right corner of the SRR; this modification causes a resonance shift towards the higher frequency at 11.97 GHz that covers the X frequency band shown in Fig. 4b. Later, an I-like segment is designed at the resonator's center, interconnected between two G-like shapes. The length of the I-like shape plays a vital role in increasing the electrical length and shifting the resonances frequency. These I-like segment causes earlier shift resonance of S 11 at 4.25 GHz and resonance of S 21 at 4.27 GHz, respectively, are shown in Fig. 4a and b. All shapes are interconnected by adding two square-shaped metallic strips on two sides of the I-like shape. These two square-shaped metallic strips allow the current to flow from one G-like shape to another under the effect of H-field. Finally, a parametric sweep is performed to achieve lower resonance at the 3.09 GHz, including the S-band frequency range shown in Fig. 4b. Table 2 summarizes the response of S 21 . . com/ produ cts-servi ces/ simul ia/ produ cts/ cst-studio-suite) 30 . www.nature.com/scientificreports/ Step 1 Step 2 Step 3 Step 4 Proposed unit cell

E-field, H-field, and surface current analysis
The analysis of E-field, H-field, and surface current distribution at different resonance frequencies help to understand the structure. The electromagnetic waves penetrate the unit cell during the simulation process and respond to S 11 and S 21 . At 3.94 GHz frequency, a high-intensity E-field is observed at the bottom and top resonator's corners, as displayed in Fig. 5a. The higher density, lower high, medium, lower medium, then less intensity of the E-field, H-field, and surface current are indicated by red, orange, yellow, green, and blue colors. The design saw medium E-field intensity at the lower portion of the inverse G-like shape, inverse G-like shape, and I-like shape. Moreover, the same phenomenon has happened in the top and bottom parts of the I-like segment shown in Fig. 5b. On the other hand, at 11.175 GHz, the strength of the electric field (E) over the entire resonator is decreased, and a lower electric field (E) intensity is observed over the SSRR. Nevertheless, a comparatively high intensity electric field (E) is perceived at the top and bottom of the I-like shape and then at the bottom of inverse G-like shape, as displayed in Fig. 5c. At 3.94 GHz, an energetic magnetic field (H) intensity is observed at the middle and lower left portions of the inverse G-like shape. A relatively lower magnetic field (H) intensity is noticed at the top and bottom right parts of the resonator, as depicted in Fig. 6a. Dramatically magnetic field intensity is decreased in the resonator at 8.08 GHz except for the top right portion, as presented in Fig. 6b. However, High magnetic field intensity is observed around the I-like shape at 11.175 GHz, as illustrated in Fig. 6c. Later, the circulation and distribution of surface current are analyzed.A strong surface current is observed at 3.94 GHz in the lower-left portion of the inverse G-like shape. The medium surface current intensity is observed in the middle of the I-like shape. Low, medium intensity is observed at the lower portion of the SSRR presented in Fig. 7a. Consequently, a well-distributed surface current is observed at the entire resonator at 8.08 GHz. Still, a strong current is observed in the middle right part of the resonator, presented, in Fig. 7b. A strong surface current distribution was at 11.75 GHz observed around the I-like shape and bottom portion of the inverse G-like shape, as presented, in Fig. 7c.

Equivalent circuit modeling
The metamaterial structure contains metal strips with split gaps that exhibit inductive and capacitive effects. The metal strips induce the inductance, whereas split gaps contribute to the capacitance of the structure. Thus, the whole structure resembles an LC circuit that exhibits resonance, with resonance frequency (f) having the following relation 31 : √ LC Table 2. Transmission Coefficient (S 21 ) of successive steps for the proposed unit cell.

Substructure
Resonance frequency (GHz) Resonance peak (dB) Covering bands Step 1  here, ε 0 and ε r denote the free space permittivity and relative permittivity, whereas A and d indicate the metal strip area and split distance. The inductance of the resonator ring can be computed by applying the transmission line principle 32 : Here, the correction factor, K g = 0.57 − 0.145 ln w ′ h ′ , in which w ′ and h ′ are represent the substrate width and thickness, respectively. Moreover, t, l, and w denote the thickness, length, and width of microstrip lines, respectively. The projected MTM structure conceives a square outer ring that is split twice. An equivalent circuit is considered in Fig. 8, in which this outer ring is presented by a group of inductances and capacitances, namely L1, L2, L3, L4, L5, L6, L15, and L16. This inductance resembles the effects of copper for different segments of the outer rings, whereas capacitors C1 and C2 are due to the split gaps in this ring. This ring is an interconnector with two modified E-shaped metal strips that are presented with the equivalent inductances L7, L8, L9, L10, L11, L12 in distribution form like the line inductances in a transmission line. The middle of the I-like-shaped structure equates with the inductance L13, L14, and as the I-like shape is connected with two E-shaped structures, a short circuit is employed in the middle of the equivalent circuit. Moreover, inductance L17 characterizes the corresponding inductance between E outer ring and internal structure. The co-planar capacitances that exist between the outer   Figure 9 shows a comparison plot of S 21 that describes the nature of the transmission coefficient acquired from circuit simulation and MTM simulation. A close similarity is noticed between the two responses; thus, the equivalent circuit in Fig. 1 with component values replicates the MTM unit cell present in this manuscript.

Results and discussion
Extraction method of effective medium parameters. The graphical depiction of S-parameters adapted with the CST post-processing section is demonstrated in Fig. 10a Fig. 10a has depicted the response of S11 and S21 to the structure.Each lower resonance value of S21 is consistently lower than the corresponding resonance of S11. Regarding data of S-parameters is presented in Table 3. The negative relative permittivity characteristic is exhibited at 3.8-4.17 GHz, including the S-band frequency shown in Fig. 10b. After that, another negative relative permittivity characteristic is exhibited between the 7.68-8.54 GHz frequency ranges. Moreover, in the rest of these frequency ranges, the unit cell exhibited negative permittivity values between the 5.04-5.17 GHz frequency ranges with slightly lower amplitude value including C-band frequency. Furthermore, another negative permittivity is exhibited in the 10.67-11.36 GHz frequency range, including the X-band frequencies. It is observed from Fig. 10b that when the resonance of S21 occurs at that point, negative permittivity is achieved. It is observed from Fig. 10b that when the resonance of S21 occurs at that point, negative permittivity is achieved. Moreover, noticed that where the permittivity fluctuates positive to negative then permeability is positive with a continuous change within the frequency ranges, as shown in Fig. 10c. Figure 10c is presented the graphical representation of relative permeability. The near-zero refractive index is exhibited at 4.07-4.15 and 8.29-8.37 GHz, respectively, shown in Fig. 10d. Since the structure exhibits the negative permittivity property, the structure can be defined as ENG metamaterials. The SNG metamaterial can be used in several microwave applications, for example, to increase the antenna gain and bandwidth and filter designing 29 .
The response of S 11 and S 21 impacts the impedance of structure can be the relation between scattering parameters and effective input impedance Eq. 33 .   The normalized impedance diagram of the structure is depicted in Fig. 11. As depicted in Fig. 11, the real part of the normalized impedance is positive, whereas the imaginary part is negative. The real part of the impedance approaches near zero in the vicinity of the frequency 3.93 GHz, 8 GHz, 10.94 GHz with a magnitude of 0.035, 0.03, 0.05, respectively. In contrast, two other dips of impedance spectra are observed at 5.04 GHz and 9.15 GHz with a magnitude of 0.11 and 0.15. The positive real part of impedance indicates that forward wave propagation occurs through the proposed MTM when exposed to electromagnetic waves 34 .

Study the effect of polarization and incident angle change.
This study investigated the changing effect of polarization and incident angle of the structure. The normal polarization angle and oblique polarization are presented in Fig. 12a and b is depicted the normal incident angle and oblique incident angle. The proposed structure is demonstrated the polarization insensitivity nature (0°-90°) at TE and TM mode, which is presented in Fig. 12c-d. Incident angle (θ) is investigated at 0°-60° for TE and TM modes, as displayed in Fig. 12e-f. The incident angle insensitivity is proportion to the path length, so more incident angle insensitivity can be achieved when path length is increased. Moreover, an inclusive electric field is perpendicular to the incident angle for better stability. However, the proposed structure has an excellent coupling effect with a wide polarization angle that increases the application necessities in the communication systems.
Experimental result analysis of MTM unit cell. The structure is fabricated on a 9 × 9 mm 2 FR4 substrate to validate experimentally. The transmission coefficient (S 21 ) is generated to verify the numerical result from the measurement process. The performance of the fabricated prototype is experimentally verified by using Agilent PNA Network Analyzer N5227A with three different waveguide ports, as shown in Fig. 13a. The prototype is placed between two waveguides to coaxial adapter P/N: 187 WCAS when measuring the transmission coefficient (S 21 ) from 3.95-5.85 GHz. Similarly, WG to coaxial adapter P/N: 137 WCAS and WG to coaxial  Compared to the simulation results, slight differences are observed because of these constraints; the experimental result shows a minor shifting in resonant frequency with harmonics. There are various reasons behind this mitch match, such as fabrication error, drift error, EMI noise, and signal leakage and reflection. However, simulated and measured results included the S, C, and X band frequency range.
EMR and comparative study of the proposed unit cell. Table 4 compares the proposed structure with existing work regarding the resonance frequency of transmission coefficient (S 21 ), operating band, EMR value, and the physical and electrical dimensions. EMR has significance in controlling the miniaturization of the structure . The EMR is calculated by using the relation, EMR = L Where λ represents the wavelength , and L denotes the structural length of the square-shaped unit cell. The SNG or DNG characteristics can be possible if the EMR is more than 4. The EMR value of the proposed MTM unit cell is 8.47 at 3.94 GHz resonance frequency. The electrical measurement of structure is 0.118λ × 0.118λ × 0.021 λ. From Table 4, it is observed that the dimension of Ref. 21 and Ref. 24 is less than the proposed structure dimensions. But the number of resonances is less than the proposed structure . Reference 26 covers only the Ku band with an EMR of 1.7, much less than the proposed MTM. In Ref. 35 coverage, the frequency band of X and Ku, respectively, but the EMR value is 1.4, and the dimension of this unit cell is much higher than the proposed structure . Reference 32 covers C, X, and Ku bands; even the EMR value is almost similar to the proposed unit cell, but this unit cell did not show the polarization characteristics. Compared to the existing design unit cell in Table 4, the proposed structure is expressed its its supremacy in size, maximum resonances, covering bands, and independent polarization characterization.

Conclusion
This study investigated an inverse G-like shape to enable ENG and NZI characteristics for microwave application. The electrical measurement of the structure is 0.118λ × 0.118λ × 0.021 λ, and the EMR value is 8 A widespread investigation has been executed to analyze the E-field and H-field intensity at the resonance frequency. Later, the surface current behavior (circulation and density) is also analyzed to comprehend the structure. This study also manifests the polarization angle insensitivity response of 0°-90°    www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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